Electrospray is a common method of soft ionisation in biochemical mass spectrometry (MS), since it allows the analysis of fluid samples pre-separated by liquid chromatography (LC), the ionization of complex molecules without fragmentation, and a reduction in the mass-to-charge ratio of heavy molecules by multiple charging [Gaskell 1997; Abian 1999]. It may be used in a similar way with fluid samples pre-separated by other methods such as capillary electrophoresis (CE).
The principle is simple. A voltage is applied between an electrode typically consisting of a diaphram containing an orifice and a capillary needle containing the analyte. Liquid is extracted from the tip and drawn into a Taylor cone, from which large charged droplets are emitted. The droplets are accelerated to supersonic speed, evaporating as they travel. Coulomb repulsion of the charges in the shrinking droplet results in fragmentation to ions when the Rayleigh stability limit is reached. The resulting ions can be multiply charged.
An electrospray mass spectrometer system contains a number of key elements:                An electrospray ionisation source capable of interfacing to an LC or CE system        An interface to couple ions (in preference to molecules) into a vacuum chamber        An alignment and/or observation system capable of maximising the coupling        A mass filter and detector        
Conventionally, the spray is passed from atmospheric pressure via a chamber held at an intermediate pressure. Several vacuum interfaces that use differential pumping to match flow rates to achievable pressures have been developed [Duffin 1992]. The ion optics normally consist of input and output orifices such as capillaries, capillary arrays and skimmer electrodes, and occasionally also a quadrupole lens operating as an ion guide in all-pass mode. These components are used to maximise the ratio of coupled ions to neutrals, which would otherwise swamp the chamber.
Various methods are used to promote a well-dispersed spray of small droplets and hence a concentrated flow of analyte ions. Solvent can be preferentially driven off, by direct heating [Lee 1992]. Advantages may be obtained by the use of a sheath gas flow [Huggins 1993], and nebulisation may be enhanced by ultrasound [Hirabayashi 1998].
Alignment in electrospray is not critical, and the spray may simply be directed towards the MS input. Alternatively, an off-axis spray direction may be used to promote the separation of neutrals. Co-axial lenses mounted directly on the capillary have been developed to focus the spray [U.S. Pat. No. 6,462,337]; however, there are limits to the electrode complexity that can be achieved using such simple mechanical systems.
In a conventional electrospray system, with capillaries of =100 μm internal diameter, flow rates are of the order of 1 μl min−1, and extraction voltages lie in the range 2.5 kV-4 kV. Flow rates and voltages are considerably reduced in so-called “nanospray systems”, based on capillaries having internal diameters ranging down to =10 μm [Wilm 1996]. Such capillaries are relatively easy to fabricate, and are available with a range of diameters and frits. Decreasing the capillary diameter and lowering the flow rate also tends to create ions with higher mass-to-charge ratio, extending the applicability further towards biomolecules.
Because of the reduced size of the spray cone, alignment of a nanospray source is more critical. Operation typically involves mounting the source on a micropositioner and using a video camera to observe the spray entering the vacuum inlet of an atmospheric pressure ionisation (API) mass spectrometer. Sources are sold customised for most popular brands of mass spectrometer. However, such systems are large, complex and costly.
To reduce costs, a variety of attempts have been made to integrate some of the components of nanospray ionisation sources. Ramsey and Ramsey [1997] showed that a spray could be drawn from the edge of a glass chip containing an etched capillary. Since then, integrated capillaries with in-plane flow have been demonstrated in many materials, especially plastics [Licklider 2000; Svedberg 2003]. In some cases, the fluid has been extracted from a slot rather than a channel [Le Gac 2003]; in others, from a shaped surface [Kameoka 2002]. Devices have also been formed in one-dimensional arrays. Geometries in which the flow is passed perpendicular to the surface of the chip have also been demonstrated, often by deep reactive ion etching of silicon [Schultz 2000; Griss 2002]. Such devices may be formed into two-dimensional arrays.
Almost exclusively, the advances above consist of attempts to integrate system sub-components leading up to the ion emitter. They concentrate on the fluidic part of the system, ignoring the problems of separating ions from neutrals, and of aligning the ion spray to the inlet to the vacuum system. As a result, they are not suitable for a low cost nanospray system, because accurate alignment still requires expensive positioning devices.
There is therefore a need to provide a low cost nanospray system.